A passive optical network (PON) comprises an optical line terminal (OLT) connected to multiple optical network units (ONUs) in a point-to-multi-point network. New standards have been developed to define different types of PONs, each of which serves a different purpose. For example, the various PON types known in the related art include a Broadband PON (BPON), an Ethernet PON (EPON), a Gigabit PON (GPON), a ten-Gigabit PON (XG-PON), and others.
An exemplary diagram of a typical PON 100 is schematically shown in FIG. 1. The PON 100 includes N ONUs 120-1 through 120-N (collectively known as ONUs 120) coupled to an OLT 130 via a passive optical splitter 140. In a XG-PON, for example, traffic data transmission is achieved using XGPON encapsulation method (XGEM) encapsulation over two optical wavelengths, one for the downstream direction and another for the upstream direction. Thus, downstream transmission from the OLT 130 is broadcast to all the ONUs 120. Each ONU 120 filters its respective data according to pre-assigned labels (e.g., XGEM port-IDs). A similar traffic data transmission technique is also utilized in GPON.
The splitter 140 is 1 to N splitter, i.e., capable of distributing traffic between a single OLT 130 and N ONUs 120. In most PON architectures, the upstream transmission is shared between the ONUs 120 in a TDMA based access, controlled by the OLT 130. TDMA requires that the OLT first discovers the ONUs and measures their round-trip-time (RTT), before enabling coordinated access to the upstream link. Each ONU 120 is equipped with a BER measurement unit 122.
In order to provide reliable data communication forward error correction (FEC) is applied on the data frames transmitted by the OLT in the downstream direction and frames transmitted by the ONUs in the upstream direction. A FEC is a well-known technique in data communication for correcting errors in data transmission over unreliable or noisy communication channels. A FEC is accomplished by adding redundancy (parity) bytes to the transmitted data using a code. Examples for FEC coding techniques include, for example, Reed-Solomon (RS), Bose and Ray-Chaudhuri (BCH), low-density parity-check (LDPC) coding, and the like.
The utilization of FEC in a GPON is optional. However, when in use, only a Reed-Solomon RS(255,239) code is permitted. The length (size) of the data section of each FEC codeword is 239 bytes, and the number of parity bytes of this codeword is 16 bytes. A GEM frame's structure is defined in the GPON standard ITU-T G.984.3, referenced herein for the useful understanding of the background.
FIG. 2 illustrates an XG-PON1 downstream physical (PHY) frame 200 that includes a physical synchronization block (PSBd) portion 210, a XGPON transmission convergence layer (XGTC) header 220, and a XGTC payload 230. The duration of a downstream PHY frame 200 is 125 microsecond.
The PSBd 210 defines certain provisions for the transmission of the downstream PHY frame 200. The XGTC header 220 includes a predefined number of bandwidth (BW) maps 221 and physical layer operations and maintenance (PLOAM) messages 222. The XGTC header 220 also includes a HLend field 223, which designates the number of BW maps and PLOAMs for the current frame.
In the downstream direction, the XGTC payload 230 includes a plurality of XGEM frames 231, each includes a XGEM header and payload. The XGTC header 220, and a XGTC payload 230 are coded using the RS(248, 216) code, with a data codeword of 216 bytes and 32 parity bytes. The structures of XGEM frame and XG-PON1 downstream PHY frame are specified in the XGPON standard ITU-T G.987.3, referenced herein for the useful understanding of the background.
In the upstream direction, XGEM frames can be transmitted to the OLT either as FEC-enabled burst series or a FEC-disabled burst series. In the FEC-enabled mode, data is FEC coded using the RS(248, 232) code. The determination of whether the upstream burst should be FEC-enabled is performed by the OLT. The OLT informs the ONUs of the selected mode (FEC enabled/disabled) using a profile downstream PLOAM message. This message associates, among other parameters, a FEC enabled/disabled parameter to every burst profile. When the OLT sends bandwidth allocation structures to the ONU, each bandwidth grant carries, among other parameters, a reference to one of the defined burst profiles, thus mandating whether FEC is used in each burst transmission by each ONU. The XG-PON standard specifics that when the FEC is enabled, an ONU can encode the transmitted burst data only using the RS(248,232) code.
The FEC correction code permitted to be utilized is according to the maximum bit error ratio (BER) that can be tolerated. Specifically, physical media dependent layer (PMD) parameters specified in ITU-T G.987.2, section 9.2, are defined relative to a bit error rate (BER) of 10−3 in the downstream direction and 10−4 in the upstream direction. The PMD parameters include, among others, permitted ODN (optical distribution network) attenuation range, maximum fiber distance, line codes, masks of transmitter eye diagrams, minimum and maximum mean launched power, minimum extinction ratio, minimum receiver sensitivity, and more. Thus, when the values set to these parameters are met, the resulting BER is expected to be bounded by 10−3 in the downstream direction or 10−4 in the upstream direction. However, these error rates are not acceptable for a reliable data link, therefore a FEC is needed.
The BER level at the output of the FEC decoder (that is, after FEC correction is applied), is specified by XG-PON standard ITU-T G.987, section sections 5.2-5, to be 10−12 or better. The FEC code specified by the respective standard can be used to bridge the gap between the BER provided by the PMD layer and the BER required by the upper layers of the XG-PON or GPON protocol.
The single FEC code, specified by the above-referenced standards, defined based on a theoretical network with a typical number of ONUs and specific conditions on the optical fiber. However, this limits the bandwidth utilization and reliability of the network. For example, in an XG-PON deployment where a small number of ONUs are installed and the range between the OLT to the ONUs is relatively short, less restricted FEC codes can be utilized where less parity bytes are transmitted. Thus, increasing the bandwidth utilization. On the other hand, when the fiber conditions are degraded, either permanently as a result of an intentional assumptions taken in the course of the network design, e.g., installation of a large number of ONUs, having a long distance between the OLT and ONUs, and/or using poor quality optical components, or temporarily, e.g., due to construction work in the fiber vicinity, inclement weather conditions, performance degradation due to aging, or physical damage to the ODN, a stronger FEC code should be used to improve transmission reliability.
It would be therefore advantageous to provide a solution for adaptively selecting and applying the FEC to improve the efficiency of the PON.